As used in the present specification, the expression "analog memory device" corresponds to a device into which analog information (information such as a television image, which has a gradation corresponding to the brightness of a picture signal) can be written in conformity with the density of the information signal in analog form.
In general, ferroelectric material has the property that a domain having the same sign or polarity as that of the voltage applied to the material grows therewith. When the applied voltage is interrupted in the course of a change of the domain of the ferroelectric material, the growth of the domain ceases. Accordingly, the charge-versus-electric field variation of the ferroelectric material is not discontinuous, but is continuous. In other words, the state of polarization of the ferroelectric material can take on any value between both positive and negative saturation charge levels.
In ferroelectric material, however, the portion which has saturation charges is in a state of saturation polarization, which exhibits a value determined in dependence upon the type of material. This means that the spontaneous polarization of the ferroelectric material is closely associated with the crystal structure and corresponds to a material constant. Accordingly, the magnitude of the spontaneous polarization of the unit cell of ferroelectric crystal material has a fixed value, while the polarity thereof will change in accordance with the sign of the applied voltage which exceeds a threshold value. Namely, even when the applied voltage is reduced to zero at any point in the midst of a polarization reversal (reversal of the polarity of spontaneous polarization), the spontaneous polarization of the unit cell will remain unchanged unless a voltage having an opposite polarity to that of the polarization and exceeding the threshold voltage is applied to the crystal. Namely, the spontaneous polarization of the ferroelectric material is considered to be bistable.
The spontaneous polarization of a unit cell is bistable in the above-described type of ferroelectric material, yet since the ferroelectric material has a domain structure, the polarization (charge) of the entire ferroelectric crystal can take on an arbitrary value between that of a positive saturation value and that of a negative saturation value (charge). Namely, depending upon the sign of the applied voltage, domains of both the same and opposite signs will grow and vanish, respectively (this phenomenon is termed domain switching). The mode in which the entire ferroelectric crystal becomes saturated at a positive or negative domain in correspondence with the applied voltage is called full switching, while the mode in which only a portion of the crystal has positive or negative domains and the other part has domains of the opposite sign is called partial switching.
When a digital memory is to be employed in a memory device employing ferroelectric material, the switching mode of full switching is adopted. When an analog memory is to be constructed of such ferroelectric material, partial switching is employed.